The present invention relates to an optical acquisition apparatus for use with an image capturing and recognition system. In particular, the present invention includes an optical acquisition apparatus for obtaining high contrast, low distortion images of patterned objects.
Patterned object recognition systems are becoming common in industrial and commercial settings and have a variety of uses. For example, such systems can be used in scanners for the scanning of text, drawings, and photographs. Recently, manufacturers have been attempting to reduce costs associated with pattern recognition systems to make them more viable for consumer use. One such consumer application for pattern recognition systems includes fingerprint acquisition and recognition. Such a system is useful, for example, to enhance computer security by reading a potential user""s fingerprint to compare with the fingerprints of users authorized to use the computer or access certain files or functions of the computer. Such a system could, for example, take the place of a security system that uses a login name and password.
The first thing such a fingerprint recognition system, or any pattern recognition system, must be able to do is to accurately acquire the fingerprint, or other pattern, for analysis. A number of mechanisms exist for such acquisition of pattern data. For example, U.S. Pat. No. 3,975,711; 4,681,435; 5,051,576; 5,177,435 and 5,233,404 all disclose apparatuses for acquiring an image of a patterned object.
FIG. 1 shows a schematic diagram of one such prior art optical fingerprint capturing and recognition system. In FIG. 1, an optical recognition system 108 includes an light source 112, an optical triangular prism 110, a lens assembly 114, an image sensor 116, and a storage and processing unit 125. The prism 110 includes an imaging surface 118, a light receiving surface 120, and an viewing surface 122. Imaging surface 118 is the surface against which a patterned object, such as a fingerprint, is placed for imaging. The light source 112, which may, for example, be a light emitting diode (LED), is placed adjacent to light receiving surface 120 and generates incident light 124 that is transmitted to the optical prism 110. The optical prism 110 is an isosceles right triangle, with the angle opposite the imaging surface 118 being approximately 90 degrees and the other two xe2x80x9cbasexe2x80x9d angles (that is, the two angles of an isosceles prism that are equal) each being approximately 45 degrees.
Generally, incident light 124 strikes imaging surface 118 at an angle 126 with the incident surface normal line 115. Angle 126 is greater than the critical angle 128. In general, a critical angle is measured between an incident light ray and a normal line to a surface. Above the critical angle, the incident light will undergo total internal reflection off the surface, and below the critical angle the incident light will pass through the surface. Accordingly, critical angle 128 is the angle with the normal line to the imaging surface 118 above which incident light will totally internally reflect from imaging surface 118 and pass out of prism 110 as reflected light 130 through viewing surface 122. Reflected light 130 passes through lens assembly 114 located adjacent to viewing surface 122. Lens assembly 114 may contain one or more optical lenses. Thereafter, light from lens assembly 114 is captured by image sensor 116. Image sensor 116, which may, for example, be a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, captures optical light images and converts them to electrical signals. Such image sensors are well known to those skilled in the art. The electrical signals are then transmitted to the storage and processing unit 125.
Storage and processing unit 125 may include a memory unit, a processor and an analog to digital converter (not shown). The analog to digital converter converts the analog electrical signals from the image sensor 116 into digital data. The memory is used to store the digital data and algorithms for comparing a captured fingerprint image with a stored fingerprint image. The processor compares the captured digital data with data previously stored in memory based on an algorithm for comparing such data. The processor may also analyze the captured digital data for purposes different from comparison with stored data. Such storage and processing units are known to those skilled in the art and can include standard personal computers equipped with appropriate software. Algorithms for processing and comparison of image data are disclosed, for example, in U.S. Pat. Nos. 4,135,147 and 4,688,995 each of which is incorporated in its entirety by reference.
When a fingerprint is placed on the optical prism""s imaging surface 118, ridges 111 of the fingerprint contact imaging surface 118, and valleys 109 of the fingerprint remain out of contact with imaging surface 118. Thus, in fingerprint valleys 109 incident light 124 entering the optical prism 110 from the light source 112 undergoes total internal reflection at imaging surface 118 if the incidence angle of the incoming light exceeds the critical angle of the optical prism 110. However, at ridges 111 of a fingerprint some of incident light 124 is absorbed and scattered off the fingerprint ridge. As used herein, the term xe2x80x9cscatteredxe2x80x9d indicates light which, after striking an irregular surface, is radiated or irregularly reflected off the irregular surface in multiple directions.
As a result of this scattering and/or absorption, there is less than total internal reflection of incident light 124 at fingerprint ridges 111. Thus, the intensity of reflected light 130 leaving prism 110 from the valleys 109 of a fingerprint is of greater intensity than reflected light 130 leaving prism 110 from ridges 111. The lower intensity reflected light 130 from ridges 111 translate into darker regions to indicate the presence of an object at the point of incidence between the light beam and the fingerprinting surface. Conversely, higher intensity reflected light 130, such as that which undergoes total internal reflection, translates into brighter regions to indicate the absence of an object at the point of incidence between the incident light 124 and the imaging surface 118. This allows distinguishing the darker fingerprint ridges 111 from the relatively brighter fingerprint valleys 109. Because absorption of incident light at fingerprint ridges 111 is primarily responsible for creating a fingerprint image, system 108 is referred to as an xe2x80x9cabsorptionxe2x80x9d imaging system.
The above described system allows capturing an optical fingerprint image and processing the electrical representation of the optical fingerprint image. However, in regions of fingerprint ridges 111, incident light 124 still undergoes some total internal reflection and some scattering in a direction parallel to reflected light 130. Thus, the difference in intensity between reflected light 130 from fingerprint valleys 109 and fingerprint ridges 111 can be relatively low. That is, the contrast between fingerprint ridges 111 and valleys 109 in the fingerprint image can be relatively low. This can make image acquisition, processing, and comparison relatively difficult.
Additionally, the optical recognition system 108 tends to be relatively large due to the relatively large distance between the optical prism 110 and the lens assembly 114. The large distance between the optical prism 110 and the lens assembly 114 is caused by the fact that a fingerprint in imaging surface 118 is likely to be larger than the first lens in lens assembly 114. Thus, if lens assembly 114 is placed relatively close to viewing surface 122, lens assembly 114 will probably not capture the fingerprint image at points near the edges of the fingerprint. Therefore, a relatively large distance between the optical prism 110 and the lens assembly 114 is desirable in system 108 because it can provide better imaging near fingerprint edges. Thus, making image acquisition system 108 relatively compact can be problematic. Additionally, a relatively large distance between viewing surface 122 and lens assembly 114 can cause loss of contrast in the fingerprint image due to light interference.
Further, a phenomenon known as trapezoidal distortion can occur in pattern acquisition system 108. Trapezoidal distortion in an imaging system has the effect of making the image of a square created by the system appear as a trapezoid. FIG. 2 is a schematic illustration showing why trapezoidal distortion arises in image acquisition system 108. Incident light 124 from light source 112 enters prism 110 and reflects off of imaging surface 118, imaging object AB. Reflected light 130 then passes out of viewing surface 122 and to lens assembly 114 at points Axe2x80x2 and Bxe2x80x2 to form object Axe2x80x2Bxe2x80x2. Viewing object AB through viewing surface 122, object AB would appear to be located at an xe2x80x9capparent imagexe2x80x9d object ab. Specifically, point A appears to be at point a, a distance aaxe2x80x2 from viewing surface 122 and point B appears to be at point b, a distance bbxe2x80x2 from viewing surface 122. The distance that an apparent image of an object appears from viewing surface 122 is given by the actual distance the object is from viewing surface 122 divided by the index of refraction n of prism 110. Specifically, the distance aaxe2x80x2 is given by:
aaxe2x80x2=Aaxe2x80x2/n,
where xe2x80x9cnxe2x80x9d is the index of refraction of prism 110. Similarly,
xe2x80x83bbxe2x80x2=Bbxe2x80x2/n.
Trapezoidal distortion occurs when the light path length from the apparent image of an object to the lens plane of lens assembly 114 is different for different parts of the imaged object. Specifically, trapezoidal distortion occurs in system 108 because the distance aAxe2x80x2 is longer than the distance bBxe2x80x2. As the above equations make clear, trapezoidal distortion can only occur when light is passed through an object having an index of refraction that does not equal 1 (assuming the object is in air having an index of refraction of n=1).
To correct this distortion, prior art manufacturers have tilted the lens plane 107 of lens assembly 114 and image sensor 116 to increase the distance bBxe2x80x2 and decrease the distance aAxe2x80x2 to a point where the two distances are approximately equal. However, it is a property of an isosceles right prism (that is, a triangular prism in which the base angles measure approximately 45 degrees and the non-base angle, or apex angle, measures approximately 90 degrees), that reflected light 130 exits prism 110 substantially normal to viewing surface 122. That is, no refraction of reflected light 130 occurs as it exits viewing surface 122. Further, generally, the larger the angle of incidence on a surface of a transparent object, the greater the portion of incident light that is reflected from the surface. Thus, while tilting lens assembly 114 can reduce trapezoidal distortion, it also causes greater reflection of reflected light 130 off of the surface of lens assembly 114, and the surface of image sensor 116, because reflected light 130 strikes lens assembly 114 at a greater angle of incidence. This reduces the intensity of light entering image sensor 116, making image processing and comparison more difficult.
Additionally, the relative placement of light source 112 and lens assembly 114 make it possible for stray light 113 emitted by light source 112 to enter lens assembly 114. This can generate additional background xe2x80x9cnoisexe2x80x9d light which can further reduce the quality of an captured image and make image processing more difficult.
To overcome some of the difficulties associated with the type of absorption image acquisition system described above, acquisition systems have been designed which are based primarily on scattering mechanisms rather than absorption mechanisms. One such acquisition system is disclosed by U.S. Pat. No. 5,233,404 issued to J. Lougheed et al. on Aug. 3, 1993 (Lougheed et al.). FIG. 3 is a schematic diagram illustrating the image acquisition portion of the apparatus disclosed by Lougheed et al. As shown in FIG. 3, a prior art image acquisition system 208 includes a trapezoidal prism 210, a light source 212, a lens assembly 214 and an image sensor 216. The trapezoidal prism 210 includes at least an imaging surface 218, a light receiving surface 220, and a viewing surface 222.
The imaging surface 218 is the surface against which an object to be imaged, such as a fingerprint, is placed. The light source 212 is located adjacent to and facing the light receiving surface 220 which is substantially parallel to imaging surface 218. Thus, incident light 224 emitted by light source 212 projects light through prism 210 and onto imaging surface 218 at an angle which is generally less than the critical angle 228 of imaging surface 210. Therefore, in the valleys 209 of a fingerprint placed against imaging surface 218 where the fingerprint is not in contact with imaging surface, total internal reflection does not occur and incident light 224 passes through imaging surface 218. At points where fingerprint ridges 211 are in contact with imaging surface 218, incident light 224 strikes the fingerprint ridge to generate scattered (or equivalently, irregularly reflected) light 230. Scattered light 230 propagates back into prism 210 in substantially all directions including the direction of lens assembly 214, located adjacent to viewing surface 222. Scattered light passes through viewing surface 222 and into lens assembly 214 to be detected by image sensor 216, which, as above, can be a CCD, CMOS or other type of detector.
In the region of a fingerprint valley 209, incident light 224 passes through imaging surface 218. And, in the area of a fingerprint ridge 211, incident light 224 scatters off imaging surface 218 to be picked up by lens assembly 214 and image sensor 216. Accordingly, the image of the fingerprint is relatively bright at fingerprint ridges 211 and relatively dark at fingerprint valleys 209. Because scattered light 230 is picked up by the image sensor 216, this type of system is referred to as a xe2x80x9cscatteringxe2x80x9d system.
The difference in intensity between the ridges and valleys in a fingerprint image created by such a scattering system can be greater than the difference in intensity between the ridges and valleys of a fingerprint image created in an absorption system as shown in FIG. 1. As a result, the fingerprint image created by such a scattering system can display higher contrast between fingerprint ridges and valleys than an image created by an absorption system. Thus, the image can be more accurately acquired by the image sensor 216. This can reduce errors in subsequent fingerprint comparisons performed by the system. However, a trapezoidal prism such as prism 210 can be more expensive to manufacture than a triangular prism such as prism 110, shown in FIG. 1. This is because, among other reasons, there is an extra surface to polish. This can increase the price of an imaging system such as imaging system 208, making it less viable for consumer use. Further, a trapezoidal prism such as prism 210 which is large enough to be used for fingerprint imaging can be larger than a similarly suited triangular prism. Thus, use of a trapezoidal prism such as prism 110 can cause an imaging system to be relatively less compact.
Additionally, image acquisition system 208 can cause trapezoidal distortion of a fingerprint image in a manner similar to that of image acquisition system 108. This is especially the case if imaging surface 218 and viewing surface 222 form an angle with each other of approximately 45 degrees. If this is the case, then image acquisition system 208 will cause trapezoidal distortion for the same reasons, discussed above, that image acquisition system 108 does. Such an image acquisition system using a trapezoidal prism having a 45 degree angle between the imaging surface and viewing surface is disclosed, for example, in U.S. Pat. No. 5,210,588.
As the above discussion makes clear, there is a need for improved image acquisition apparatus for use with patterned object recognition systems. Specifically, an image acquisition apparatus that produces a high contrast, low distortion image would be desirable. Additionally, the apparatus should be relatively compact. Also, the apparatus should be relatively low cost to manufacture, making it affordable for consumer use.
The present invention includes a compact image acquisition apparatus which produces a high contrast, low distortion image and which can be relatively low cost to manufacture. The apparatus includes a light refractor having an imaging surface against which a patterned object is to be placed, a light entrance surface, and a viewing surface. The light entrance surface is adjacent to the imaging surface and allows light to enter the refractor. The viewing surface is also adjacent to the imaging surface and an image of the patterned object is projected through the viewing surface. The apparatus also includes a focusing lens adjacent to the viewing surface for receiving and focusing an image of a patterned object. A light source is located adjacent to the light receiving surface and emits incident light which enters the refractor to create an image of the patterned object at the viewing surface. The focusing lens then focuses the image. The light source is positioned such that the light emitted therefrom strikes at least one other surface before striking the imaging surface. In this way, an image from the imaging surface and projected through the viewing surface is generated by substantially all scattered light. Such a scattered light image is advantageously relatively high contrast and evenly illuminated.
In a second aspect of the present invention, the refractor is an isosceles triangular prism having base angles which are greater than 45 degrees. Additionally, the lens plane of the focusing lens is tilted with respect to a plane defined by the viewing surface. In this way, trapezoidal distortion in an image of the patterned object is advantageously reduced.
In a third aspect of the present invention, an apparatus for forming an image of a patterned object includes a first lens, an objective lens or lens assembly, and a light source. The first lens includes an imaging surface against which a patterned object is to be placed, and a viewing surface opposite to the light entrance surface, through which an image of the object is projected. The first lens also includes a light receiving surface adjacent to the imaging surface. The apparatus further includes a light source for projecting incident light into the lens. The light source is located adjacent to the light entrance surface to project incident light between the imaging surface and the viewing surface. The incident light can undergo total internal reflection between the imaging surface and the viewing surface without passing through the viewing surface. In this way an image of the patterned object projected through the viewing surface is generated by substantially all scattered light. The apparatus also includes an objective lens or lens assembly adjacent to the viewing surface which focuses the image of the patterned object projected through the viewing surface.
The imaging surface of the first lens can be concave to better fit the contour of a fingerprint placed on the imaging surface. Additionally, the first lens and objective lens can be formed unitarily as a single piece. This can ease assembly, reduce manufacturing costs, and allow the image acquisition apparatus to be more compact.
In a fourth aspect of the present invention, an apparatus for forming an image of a patterned object includes a triangular prism, a focusing lens and a light source. The triangular prism includes an imaging surface, a light receiving surface adjacent to the imaging surface and a viewing surface adjacent to the light receiving surface. The lens is adjacent to the viewing surface and is for receiving and focusing an image of the patterned object. The light source is for projecting incident light into the triangular prism and is located adjacent to the light receiving surface to project light between the imaging surface and the viewing surface. Most of the incident light undergoes total internal reflection between the imaging surface and the viewing surface without passing through the viewing surface. In this way, an image of the patterned object projected through the viewing surface into the objective lens is generated by substantially all scattered light from the imaging surface.
In this fourth aspect of the present invention, a first light source can be placed on a first end triangular surface of the prism and a second light source can be placed on a second end triangular surface of the prism opposite to the first triangular end surface. This configuration advantageously provides even illumination of the imaging surface to generate a relatively uniform patterned object image.
A method of generating an image of a patterned object in accordance with the present invention includes providing a light refractor having an imaging surface, a light receiving surface and a viewing surface. A patterned object is placed against the imaging surface. Incident light is projected from a light source through the light receiving surface of the light refractor and reflected off at least one surface of the refractor other than the imaging surface before the incident light strikes the imaging surface. The incident light is scattered off the imaging surface and patterned object and through the viewing surface. A lens is provided adjacent to the viewing surface and the scattered light is projected into the lens which focuses the scattered light to form an image of the patterned object.